Alpha males If good looking blokes are so desirable, why isn't the world populated with men who look like movie stars? The answer, at least if you're a fruit fly, is that desirability can have a downside.

Research by the University of Queensland (UQ) has found there is a limit to the success of even the most desirable males in a population.

The report, in this week's Proceedings of the National Academy of Sciences, follows the progress of genetically engineered male fruit flies with a combination of pheromones known to be highly attractive to females. These 'attractive' males were combined with ordinary males and females.

The researchers found the number of 'attractive' males quickly increased in the population from 12 per cent to 35 per cent of the population, but leveled off after seven generations.

Further breeding showed the attractive flies begin to decline and within just four more generations, their numbers had dropped by half.

Co-author Dr Katrina McGuigan likened it to someone who has massively built up their body using steroids. They become attractive and get the girls, but at the same time they might become impotent, she says.

"We don't know what the cost is to fruit flies. They are still able to produce offspring and live long enough, but there's some cost we can't see."

The research puts a dent in a central idea of Charles Darwin's theory - male attractiveness drives evolution.

The UQ researchers suggest an opposing force, natural selection, is at play as well, putting the brakes on sexual selection when those attractive attributes also confer some sort of disadvantage to the male.

Sex has its limits

Commenting on the paper, Associate Professor Mike Schwarz from the School of Biological Sciences at Flinders University in Adelaide says some male traits increase mating success, but may harm the survival of the male.

"While these selected male flies were highly attractive, they also suffered disadvantages - likely to be lower survival rates as larvae," Schwarz.

"Emma Hine and her coauthors therefore seem to have found the hypothesized 'brake' on the limits of sexual selection."

Also commenting on the research, Professor John Endler from Deakin University in Geelong says the study shows that divergence (evolution) requires both sexual selection (the fittest males getting the most mates) and the environment.

"There appears to be a requirement for an environmental driver of divergence of sexually selected traits and mating success," says Endler. "[We now find that] ecology determines speciation and divergence for newly discovered reasons."

McGuigan says it's one of the first demonstrations that there is a cost to being attractive.

"It also implies that those amazing sexual displays males have, such as peacock tails, have probably in part evolved not because of sexual selection, but because of changes in the environment. There has to be a habitat shift before sexual selection can start to drive evolution."